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235W Solar Panels are too high voltage for GTI, Suggestions?

Discussion in 'Renewable Energy' started by Jules_Theone, May 6, 2012.

  1. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    BUT, when I was involved in a demand side management study, your generating electricity when the utility needs it and it co-insides with the air conditioning load.
    So your not home in the day time and your generating electricity for everyone using air conditioning.

    In the evening, you have cooking and AC loads from the people coming home from work. The large consumers of AC hopefully are lowering their demand because the businesses are unoccupied.

    One of the other things we looked at was to just use an unaided element in a water heater to heat hot water. That was favorable too. Agreed, it was just taking a solar panel and feeding it to a heating element with no inverter being used at all. Probably not practical.

    If you had electric hot water, you could maximize your use of solar radiation if, for instance after you took your shower and headed to work, turn off the hot water heater until your panels are producing electricity. Allow the water heater to come on with normal mains before you get home.

    One way that really helps your cause, is to get on a peak billing system. This is really what the utility would like. e.g. A good portion of your bill might be tied to the maximum electricity that you used in say a 15 minute period. So, now your forced to play games like wash dishes and use the washing machine when you and everyone else is asleep
     
  2. Jules_Theone

    Jules_Theone Member

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    I've done some more reading and it appears there are two main types of feed-in tariff; gross feed-in and net feed-in. Gross feed-in means I would get paid for every kWh I produce no matter how much of it I consume, whereas net feed-in offsets my electricity consumption and any excess I feed back to the grid I would get paid for at the rate I would pay for it (15p/kWh currently). I drew up a spreadsheet to calculate some things easier - it seems the best thing to do is consume at the exact time I generate so if I have a server on all the time the solar panels would offset that consumption for a part of the day. For every kWh I consume which isn't offset by the solar I will be charged for.

    What I want to figure out is at what point gross would be better than net for a particular size system and / or daily consumption.
     
    Last edited: May 18, 2012
  3. Jules_Theone

    Jules_Theone Member

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    Looks like I will need to get the system installed before 1st July to get the 21p/kWh rate as after that it will go down to 13.6p/kWh which will make it not worth doing.

    See attached feed-in tariff review Apr'12, according to this document the feed-in tariff will decrease the longer I wait to install panels. I think that means that noone will bother installing residential solar panels in the future.

    Jules
     

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  4. dave

    Dave New Member

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  5. rumpfy

    rumpfy Active Member

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    Solar panels are quite non linear with respect to the load voltage /load current curve. The power output from a solar panel increases with increasing current loading until the current reaches a maximum. With increasing current loading the output voltage falls. The maximum output current/ output voltage is a function of the intensity of the sunlight.
    It is possible to buy a 'solar cell power maximiser'. This device constantly adjusts the output load on the cell to allow the cell to operate at maximum power. For the use of several sets of different cells in parallel, a separate controller for each cell should be used. The output of the maximisers can then be paralleled. Then, each cell will operate at maximum power output.
    I recently purchased a solar cell to keep a tractor battery charged. The solar cell was designed to reach maximum power at 14.5 Volt. A maximiser was not necessary because the cell was designed to charge a 12 volt battery. The cell has an internal series diode to prevent self discharge.
    Using solar cells in combination therefore needs to be done with care.
     
  6. Jules_Theone

    Jules_Theone Member

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    Yes, I was thinking I might need some kind of MPPT for each different panel as they all have different maximum power voltages. My current setup is the 80W on it's own through a 350W GTI, the 40W and two 235W panels on the other 350W GTI but it doesn't seem to be operating at maximum even though I would assume putting 510W (minus inefficiencies) into a 350W inverter would make it run at maximum but I'm only getting 17v @ 11A = 187W. Ideally I will need more DMMs and try some different combinations of panels and inverters when the sun is out again.
     
  7. Jules_Theone

    Jules_Theone Member

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    I've bought a Soladin 600 string inverter for the larger panels and it seems to be much better - 52V @ 6A DC = 312W, 1.2A AC = 294W, Efficiency: 294/312 = 94%. This was in quite bright sunshine but with the panels laying flat on my shed roof. Open circuit voltage was 71V. The MCS certified 4kWp system in being installed next week so that should be interesting.
     

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